Transient Softening Research Articles

Predicting mechanical properties of mitotic spindles with a minimal constitutive model

The mitotic spindle, crucial for precise chromosome segregation and cytoplasmic partitioning during cell division, demands stability against forces arising from chromosomal movements and thermal fluctuations. Despite its central role, the mechanical properties of spindles remain largely elusive. In this study, we delve into the mechanical properties of spindles through a comprehensive model encompassing interactions among centrosomes, microtubules, chromosomes, and molecular motors. Our model successfully reproduces the 3D self–assembly of spindles and their responses to mechanical forces. We find that the spindle exhibits viscoelastic properties, responding distinctively to stretch and compression. Rapid stretch induces transient softening of the spindle, while compression leads to temporary hardening. Based on the viscoelastic responses of spindles under constant–force and constant–displacement loadings, we propose a minimal constitutive model for the spindle structure. This constitutive model can not only accurately recapture the viscoelastic responses of spindles under stretch and compression but also predict the mechanical behaviors of spindles under constant–rate loadings and cyclic loadings, which are further verified by simulations. Therefore, our validated constitutive model can replace complex simulations, providing more interesting predictions and guidance for future experiments.

Anisotropic damage constitutive model of AA5754 aluminum alloy considering warm forming history

A multipath two-stage loading test for AA5754 aluminum alloy is carried out. In the first loading stage, the tensile tests of big samples were conducted with different preforming conditions. Then small samples were cut off from the pre-strained big samples. In the second loading stage, the mechanical properties of small samples were measured, and the influence of preforming factors such as prestrain value, temperature, strain rate and stress state on the mechanical properties of the sheets has been studied. With increasing prestrain, the yield stress of the sheet increases and the failure strain decreases linearly. The higher the preforming temperature is, the lower the yield strength of the sheet is under the same prestrain, and the greater the hardening index and failure strain. Moreover, the mechanical properties of the sheet are not sensitive to the forming strain rate and stress state. In addition, a transient softening effect appeared during the two-stage loading test. A damage constitutive model considering the history of warm forming is established by VUMAT subroutine of ABAQUS commercial software. The parameters of the constitutive model are determined by the two-stage loading experimental data and a genetic algorithm. This model can describe the transient softening and Bauschinger effect of AA5754 aluminum alloy after preforming at different temperatures. Good agreement is also observed between the stress-strain responses of AA5754 aluminum alloy after warm preforming and the predictions of the model.

Effect of relative humidity on the nonlinear elastic response of granular media

We study the influence of relative humidity (RH) on the nonlinear elastic response of granular media. Previous work has shown that the nonlinear elastic response of consolidated granular media like rocks likely arises from two distinct mechanisms; however, we do not have a clear understanding of their physical origins at the microscopic scale. Here, we conduct dynamic acousto-elastic testing (DAET) on samples of glass beads under dry (∼10%), ambient (∼60%), and humid (∼100%) conditions at room temperature and a constant static stress of 4 MPa. DAET allows us to retrieve the full nonlinear elastic response, including transient softening and hysteretic effects. We find that the elastic nonlinearity of humid samples is an order of magnitude larger than dry samples. Moreover, we find that all extracted nonlinear parameters increase with RH. This overall increase in nonlinearity is consistent with findings from previous studies and with the hypothesis that water adsorption on the grains makes the contact junctions weaker and prone to greater disturbances when subjected to dynamic stressing. Our results also suggest that, if indeed both mechanisms coexist, they are affected in a similar fashion in these glass bead samples and cannot be distinguished by varying RH.

Significant Bauschinger effect and back stress strengthening in an ultrafine grained pure aluminum fabricated by severe plastic deformation process

Bauschinger test in uniaxial tension-compression mode was carried out for the first time on the pure Al specimens having homogeneous ultra-fine grained (UFG) microstructures fabricated by equal-channel angular pressing (ECAP) and subsequent annealing processes. Significant Bauschinger stress (transient softening), Bauschinger energy parameter and their strong dependences on the tensile plastic pre-strain at the very early stage of the tensile deformation were measured in the UFG specimens, in sharp contrast to their coarse-grained (CG) counterpart. The grain size dependence of the Bauschinger effect in pure Al was qualitatively discussed in terms of the back stress arising from the formation of dislocation pile-up against the grain boundary during plastic deformation.

Imaging Elastodynamic and Hydraulic Properties of In Situ Fractured Rock: An Experimental Investigation Exploring Effects of Dynamic Stressing and Shearing

AbstractWe describe laboratory experiments to elucidate the relationship between nonlinear elasticity and permeability evolution in fractured media subjected to local stress perturbations. This study is part of an effort to measure fluid pathways and fracture properties using active‐source acoustic monitoring during fluid injection and shear of rough fractures. Experiments were conducted with L‐shaped samples of Westerly granite fractured in situ under triaxial conditions with deionized water subsequently circulated through the resulting fractures. After in situ fracturing, we separately imposed oscillations of the applied normal stress and pore pressure with amplitudes ranging from 0.2 to 1 MPa and frequencies from 0.1 to 40 Hz. In response to normal stress and pore pressure oscillations, fractured Westerly granite samples exhibit characteristic transient softening, acoustic velocity fluctuations, and slow recovery, together with permeability enhancement or decay, informing us about the coupled nonlinear elastodynamic and poromechanical rock properties. Fracture interface properties (contact asperity stiffness, aperture) are then altered in situ by shearing, which generally decreases the measured elastic nonlinearity and permeability change for both normal stress and pore pressure oscillations.

Determination of resistance spot weld failure path in ultra-high-strength press-hardened steel by control of fusion boundary transient softening

Determination of resistance spot weld failure path in ultra-high-strength press-hardened steel by control of fusion boundary transient softening

Nuclear softening expedites interstitial cell migration in fibrous networks and dense connective tissues.

Dense matrices impede interstitial cell migration and subsequent repair. We hypothesized that nuclear stiffness is a limiting factor in migration and posited that repair could be expedited by transiently decreasing nuclear stiffness. To test this, we interrogated the interstitial migratory capacity of adult meniscal cells through dense fibrous networks and adult tissue before and after nuclear softening via the application of a histone deacetylase inhibitor, Trichostatin A (TSA) or knockdown of the filamentous nuclear protein Lamin A/C. Our results show that transient softening of the nucleus improves migration through microporous membranes, electrospun fibrous matrices, and tissue sections and that nuclear properties and cell function recover after treatment. We also showed that biomaterial delivery of TSA promoted in vivo cellularization of scaffolds by endogenous cells. By addressing the inherent limitations to repair imposed by nuclear stiffness, this work defines a new strategy to promote the repair of damaged dense connective tissues.

Feeling Stressed? Piezo1-Mediated Loss of Heterochromatin Buys Time for Long-Term Adaptation

Skin and other epithelial cell layers are frequently subjected to extensive deformations, yet sustain such mechanical stress without damage. In this issue of Cell, Nava and colleagues show that stretch induces rapid loss of heterochromatin that leads to transient softening of the nucleus, which, together with long-term cytoskeletal and supracellular rearrangements, protects nuclei from DNA damage.

Transient Softening at the Fusion Boundary of Resistance Spot Welds: A Phase Field Simulation and Experimental Investigations for Al–Si-coated 22MnB5

This work gives an insight into the transient softening at the fusion boundary of resistance spot welds on hot stamped steel. Metallographic investigations and hardness mapping were combined with finite phase–field modeling of phase evolution at the fusion boundary. Saturation of weld nugget growth in the welding process was observed. For industrially relevant, long welding times, the fusion boundary of a spot weld is therefore isothermally soaked between the peritectic and solidus temperatures. This leads to a carbon segregation to the liquid phase due to higher carbon solubility and possibly to δ-Fe formation at the fusion boundary. Both results in a local carbon depletion at the fusion boundary. This finding is in good agreement with carbon content measurements at the fusion boundary and the results of hardness measurements.

Single-Impact Nonlinear Resonant Acoustic Spectroscopy for Monitoring the Progressive Alkali–Silica Reaction in Concrete

Alkali–silica reaction (ASR) is a ubiquitous cause of concrete degradation. The reaction produces an expandable gel that may cause internal over-stressing and cracking. This paper demonstrates the utility of single-impact nonlinear resonant acoustic spectroscopy (SINRAS) for monitoring the progress of ASR over the course of a standard ASR-susceptibility test. In SINRAS, the transient softening and subsequent recovery of resonance frequency due to one strong impact are analyzed. The performance of SINRAS in monitoring ASR is compared to that of multi-impact nonlinear resonant acoustic spectroscopy (MINRAS), where the gradual resonance frequency shifts caused by impacts of increasing intensity is measured. The changes in standard linear expansion and linear resonance frequency are recorded in parallel. Finally, the sensitivity of measured parameters to sample temperature is investigated. Our findings indicate that SINRAS, while being much simpler and faster to conduct, yields results that strongly correlate to those from MINRAS and even gives an additional parameter describing the rate of recovery. The extracted nonlinearity parameters exhibit good sensitivity and clearly differentiate between concrete with reactive and non-reactive aggregates. Further, this study suggests that the influence of sample temperature on the nonlinearity parameters depends on the level of ASR progression and has to be taken account.

Plastic intermittency during cyclic loading: From dislocation patterning to microcrack initiation

In metallic materials subjected to cyclic loading, strain hardening as well as fatigue crack initiation have been linked for a long time with the evolution of dislocation patterns and structures. In particular, the development of low-energy dislocation configurations such as persistent slip bands (PSBs) is considered as a precursor to crack initiation. However, the associated scenarios have been elaborated mainly from postmortem observations capturing only static pictures of dislocation patterns, while the dynamics of the problem has been somewhat overlooked. Here we analyze collective dislocation dynamics during cycling loading of aluminum using acoustic emission (AE). A strong link is revealed between dislocation patterning, cyclic hardening/softening, and the intermittency of plasticity: Plastic intermittency and dislocation avalanches rapidly decay during the initial hardening stage, in conjunction with the reduction of an internal length scale characterizing the dislocation structure. However, in nonannealed samples, a transient softening stage ensues, associated with a brutal reorganization of this structure. These initial stages of cyclic deformation illustrate the competition between two phenomena: collective dislocation dynamics, governed by long-ranged elastic interactions among dislocations, and the emergence of a self-organizing network controlled by short-range interactions and progressively inhibiting collective effects. Later on, the emergence of PSBs is accompanied by a reincrease of the AE intermittent activity. We propose that the associated AE bursts may be the signature of collective and coordinated dislocation motions along PSBs leading to the formation of incipient microcracks.

The Effect of Bauschinger Related Parameters on Material Property of Dh780 and Springback Behavior of Longitudinal Beam

The application of high strength steel on automobile becomes more and more popular. So it is necessary to study the formability. In this paper, the effect of Bauschinger was studied. The material employed in this study is DH780. The influence of Bauschinger related parameters on material property was analyzed by changing the parameter values. Then, the effect on the springback behavior was studied after the finite element analysis. The part used was a longitudinal beam used in the automobiles. The related Bauschinger related parameters were Transient Softening Rate, Stagnation Ratio, Young’s Reduction Factor and Young’s Reduction Rate. The results were achieved after the analysis. The transient softening rate K shows a significant effect on the stress-strain curves. The increase of transient softening rate K leads to the decrease of reversed stress and the increase of springback.

Semi-Analytical or Inverse Identification of Yoshida-Uemori Hardening Model

The Yoshida-Uemori combined kinematic and isotropic hardening model is widely applied to numerical prediction of spring-back during sheet metal forming process. With the experimental plastic behavior of aluminum alloy AA5182-O sheet under single cyclic loading, the semi-analytical method was presented to calibrate the parameters of Yoshida-Uemori hardening model. Meanwhile, an inverse identification method was suggested by parameter optimization for minimizing the error between the experimental and predicted results. By comparing the two methods, the Yoshida-Uemori hardening model identified by inverse method is found to be more accurate for description of the Bauschinger effect than the one identified by semi-analytical method, especially for transient softening phenomenon.

Interfacial Microstructure Evolution Due to Strain Path Changes in Sliding Contacts.

We performed large-scale molecular dynamics (MD) simulations to study the transient softening stage that has been observed experimentally in sliding interfaces subject to strain path changes. The occurrence of this effect can be of crucial importance for the energy efficiency and wear resistance of systems that experience changes in the sliding direction, such as bearings or gears in wind parks, piston rings in combustion engines, or wheel-rail contacts for portal cranes. We therefore modeled the sliding of a rough counterbody against two polycrystalline substrates of face-centered cubic (fcc) copper and body-centered cubic (bcc) iron with initial near-surface grain sizes of 40 nm. The microstructural development of these substrates was monitored and quantified as a function of time, depth, and applied pressure during unidirectional sliding for 7 ns. The results were then compared to the case of sliding in one direction for 5 ns and reversing the sliding direction for an additional 2 ns. We observed the generation of partial dislocations, grain refinement, and rotation as well as twinning (for fcc) in the near-surface region. All microstructures were increasingly affected by these processes when maintaining the sliding direction but recovered to a great extent upon sliding reversal up to applied pressures of 0.4 GPa in the case of fcc Cu and 1.5 GPa for bcc Fe. We discuss the applicability and limits of our polycrystalline MD model for reproducing well-known bulk phenomena such as the Bauschinger effect in interfacial processes.

Micro‐inertia origin of instabilities in granular materials

SummaryWithin the framework of the second‐order work theory, the onset of instabilities is explored numerically in loose granular materials through three‐dimensional DEM simulations. Stress controlled directional analysis are performed in Rendulic's plane, and a particular attention is paid to transient evolutions at the microscale. Thanks to a micromechanical analysis, the onset and development of transient mechanical instabilities is explored. It is shown that these instabilities result from the unjamming and bending of a few force chains associated with a local burst of kinetic energy. This burst of kinetic energy propagates to the whole sample and provokes a generalized unjamming of force chains. As force chains buckle, a phase transition from a quasi‐static to an inertial regime is observed. At the macroscopic scale, this results in a transient softening and a loss of controllability. After the collapse of existing force chains, the development of plastic strain is eventually stopped as new stable force chains are built.

Transient softening in lubricated sliding contacts due to strain path changes

The present work investigates microstructure evolution in pure aluminium under lubricated contact conditions. The initial wear scars were obtained in unidirectional sliding up to a pre-defined number of cycles N. After this pre-imparted severe plastic deformation, the sliding direction was reversed and the resulting microstructure was characterized using hardness measurements and microscopy techniques. The results show that immediately after reversing the strain path, the material significantly softens, as known in uniaxial tension-compression experiments. After the transient softening stage, the new microstructure becomes established and the polycrystal work-hardens again. The results obtained proof the presence of strain path change effects in metals undergoing sliding contact and highlight the importance to take these effects into account in component design in order to avoid undesired material softening.

Modeling slip system strength evolution in Ti-7Al informed by in-situ grain stress measurements

Far-field high-energy X-ray diffraction microscopy is used to asses the evolution of slip system strengths in hexagonal close-packed (HCP) Ti-7Al during tensile deformation in-situ. The following HCP slip system families are considered: basal 〈a〉, prismatic 〈a〉, pyramidal 〈a〉, and first-order pyramidal 〈c+a〉. A 1 mm length of the specimen's gauge section, marked with fiducials and comprised of an aggregate of over 500 grains, is tracked during continuous deformation. The response of each slip system family is quantified using ‘slip system strength curves’ that are calculated from the average stress tensors of each grain over the applied deformation history. These curves, which plot the average resolved shear stress for each slip system family versus macroscopic strain, represent a mesoscopic characterization of the aggregate response. A short time-scale transient softening is observed in the basal 〈a〉, prismatic 〈a〉, and pyramidal 〈a〉 slip systems, while a long time-scale transient hardening is observed in the pyramidal 〈c+a〉 slip systems. These results are used to develop a slip system strength model as part of an elasto-viscoplastic constitutive model for the single crystal behavior. A suite of finite element simulations is performed on a virtual polycrystal to demonstrate the relative effects of the different parameters in the slip system strength model. The model is shown to accurately capture the macroscopic stress-strain response using parameters that are chosen to capture the mesoscopic slip system responses.

Critical hardening rate model for predicting path-dependent ductile fracture

A new phenomenological framework for predicting ductile fracture after non-proportional loading paths is proposed, implemented into FE software and validated experimentally for a limited set of monotonic and reverse loading conditions. Assuming that ductile fracture initiation is imminent with the formation of a shear band, a shear localization criterion in terms of the elastoplastic tangent matrix is sufficient from a theoretical point of view to predict ductile fracture after proportional and non-proportional loading. As a computationally efficient alternative to analyzing the acoustic tensor, a phenomenological criterion is proposed which expresses the equivalent hardening rate at the onset of fracture as a function of the stress triaxiality and the Lode angle parameter. The mathematical form of the criterion is chosen such that it reduces to the Hosford–Coulomb criterion for proportional loading. The proposed framework implies that the plasticity model is responsible for the effect of loading history on ductile fracture. Important non-isotropic hardening features such as the Bauschinger effect, transient softening and hardening stagnation must be taken into account by the plasticity model formulation to obtain reasonable fracture predictions after non-proportional loading histories. A new comprehensive plasticity model taking the above effects into account is thus an important byproduct of this work. In addition, compression–tension and reverse-shear experiments are performed on specimens extracted from dual-phase steel sheets to validate the proposed plasticity and fracture model.

On the evaluation of the Bauschinger effect in an austenitic stainless steel—The role of multi-scale residual stresses

In this work, a physically based self-consistent model is developed and employed to examine the microscopic lattice response of pre-strained Type 316H polycrystalline austenitic stainless steel subjected to uniaxial tensile and compressive loading. The model is also used to explain the Bauschinger effect observed at the macroscopic length-scale. Formulated in a crystal based plasticity framework, the model incorporates detailed strengthening effects associated with different microstructural elements such as forest dislocation junctions, solute atoms and precipitates on individual crystallographic slip planes of each individual grain within the polycrystal. The elastoplastic response of the bulk polycrystal is obtained by homogenizing the response of all the constituent grains using a self-consistent approach. Micro-plasticity mechanisms and how these influence the Bauschinger effect are illustrated in terms of the role of residual stresses at different length-scales. Overall, predictions are in good agreement with experimental data of the Bauschinger effect and the corresponding meso-scale lattice response of the material, with the latter measured by neutron diffraction. The results demonstrate that transient softening of the material is related to residual stresses at different length scales. In addition, the (Type III) residual stress at the micro-scale slip system level extends the strain range over which the tensile and compressive reloading curves of the pre-strained material merge.

In-situ neutron diffraction during biaxial deformation

A change in strain path may have a significant effect on the mechanical response of metals. In order to understand or even predict the macroscopic behaviour under such conditions a detailed knowledge on the microstructural evolution is crucial. Yet relatively little work has been done to quantify and understand how the inter- and intragranular strains are affected during a change in strain path. In this work we present a new multiaxial deformation rig that allows performing in situ proportional and non-proportional loading under neutron diffraction. We demonstrate the capabilities of this new setup for the case of a 316 L stainless steel. We show that the nature and magnitude of intergranular strain strongly depends on the applied stress state and demonstrate that micro yielding and internal strain recovery are responsible for the observed transient softening during a 90° strain path change. We anticipate that this new characterization method will provide previously inaccessible microstructural data that can serve as input for benchmarking current state-of-the-art crystal plasticity models.